PULSE WIDTH MODULATED FANS
High frequency pulse width modulated (PWM) fans perform at low noise levels by eliminating the power switching noise of the fan.
The motor commutation noise is eliminated with high frequency PWM as the coils of the fan are being switched with a frequency above 20 kHz, which is outside the human audible range. The speed of the fan is controlled by an external PWM signal. The PWM ramps up linearly from a 20% duty cycle up to the fan’s maximum rated speed.